1. Field of the Invention
This invention relates to a method for frequency-shifting the modulus of the transfer function of a digital filter having a transversal structure and also relates to devices for carrying out said method.
Filters for the frequency of electric signals are employed in many fields of electronics and especially for processing radar signals, for example in order to eliminate fixed echoes or in order to detect echoes having predetermined characteristics such as a radial velocity. This mode of processing is employed, for example, in coherent Doppler radars having constant ambiguous velocity pulses which make it possible by virtue of the Doppler effect to detect the moving obstacles which give rise to radar signals of small amplitude in the midst of stationary obstacles corresponding to radar signals of large amplitude. In fact, in these pulse radars, the waves received after reflection from moving obstacles are affected by a phase which varies from one repetition period to the next whereas the waves received from the stationary obstacles are not subject to such a phase-shift variation. In consequence, the signals corresponding to moving obstacles have, after demodulation, components which vary sinusoidally at a frequency fd called the Doppler frequency which is related to the radial velocity v and to the wavelength e of the radar by the formula Fd=2v/e. The signals which correspond to stationary obstacles have a constant amplitude and their spectrum is constituted by a series of discrete lines at the frequencies 0, Fr, 2Fr, . . . nFr, Fr being the repetition frequency of the emitted pulses. Moreover, the spectrum of signals corresponding to moving obstacles is composed of discrete lines of the type mFr -+Fd.
As may accordingly be understood, it is possible to eliminate the signals corresponding to the stationary obstacles by employing a filter for removal of fixed echoes which does not permit transmission of signals having a frequency 0, Fr, 2Fr, . . . nFr. It is also desirable in certain radars such as air traffic surveillance radars to eliminate the moving obstacles which have low Doppler velocities with respect to the velocities of the echoes of interest such as clouds, for example, or else fluctuating stationary obstacles which have a certain Doppler velocity such as trees shaken by the wind. These different low-velocity parasitic echoes are better known as "clutter".
These examples show that it would be an advantage to provide in the radar field filters having a function of transfer along the axis of frequencies which could be easily and simply modified so as to eliminate not only the fixed or pseudo-fixed echoes but also those whose radial velocity is very different from that of the targets of interest.
2. Description of the Prior Art
In radars, filters are frequently designed in digital form. In other words, the signals to be filtered are sampled at a frequency equal to the repetition frequency of the pulses emitted by the radar, whereupon the amplitude of the samples is coded in order to obtain a succession of digital codes. Finally, these codes are multiplied by coefficients having values which define the characteristics of filtering to be obtained.
These so-called digital filters are formed, for example, by means of memories placed in series which record in each case the codes of all the samples of a repetition period, by means of multiplier circuits placed at the output of said memories in order to multiply the codes read from the associated memory by a suitable coefficient and by means of an adder circuit for summing the codes resulting from the multiplications.
Digital filters of this type can be employed in fields other than radar, for example in the field of high-fidelity signals, especially when such signals are in digital form. In these fields, there also exists a need to modify the transfer function of the filters on the axis of frequencies without thereby modifying the structure of the filters to any extensive degree, for example in order to extend the rejection band of the filter.